Known back-pressure valves have a coil spring with a smaller diameter than the sealing disc, the coil spring resting full-face against the disc area. Although the sealing disc may swivel around a central transversal axis to a certain degree due to the flexibility of the spring, it is seen that it can move essentially only in an axial direction. Other sealing discs have at least four radially protruding guiding ribs which provide a guide along the wall of a casing for axial motion only of the disc. The flow resistance and hence the loss of pressure with an open back-pressure valve is relatively important in the pipe system concerned.